Broadly speaking, the present invention refers to a novel process of preparing presensitized plates by means of the use of metallic cobalt for the preparation of layers that are in themselves adherent to light sensitive materials and are useful in preparing said presensitized planographic printing plates without the need of adhesives and, more particularly, it is related to improved presensitized printing plates having as and adhesive for the light sensitive coating, a layer of cobalt applied on the outer working surface thereof.
As is well known in the art of manufacturing presensitized planographic printing plates, particularly those in which the light sensitive coating will serve as the lipophilic layer of the lithographic pair, the provision of a light sensitive coating that, while preserving its properties of ready development, will not cease to possess qualities of hardness, firm adhesion, dot fidelity and wear resistance to be useful as a lipophilic working layer in printing operations, has always been a problem of very difficult solution. In this respect, it is well known that, for optimally curing a light sensitive coating in order for it to have good qualities of hardness, firm adhesion, dot fidelity and wear resistance, it must be cured to a point which is located slightly below the lower threshold value of radiant energy which is necessary to accomplish complete polymerization or depolymerization thereof, depending on the type of light sensitive material used.
However, if a presensitized plate is sold with the light sensitive coating previously cured to the above mentioned point, said plate will show the drawback that, when applying the developer or the remover, according to the type of plate which is to be treated, these materials do not fully remove the desired areas of the light sensitive layer, because the same have been hardened to such a degree by the curing operation, that residues are always left covering and masking portions of the hydrophilic working layer.
Thus, if the sensitive coating is used when applied to an outer surface of a multimetallic printing plate wherein the hydrophilic layer is the outer layer, even when the consecutive developing and etching operations carried out on the outer metal layer of the plate may be capable of fully removing the coating affected by the developer, by entrainment with the etching fluid, the complete processing of the plate will not be efficient, because the remover which must be furtherly applied to the resist not affected by the developer in order to expose the non-etched hydrophilic metal layer, is not able to completely remove said resist, thereby leaving lipophilic residual areas that will not permit a clean operation of the printing plate.
If on the contrary, the light sensitive coating serves itself as a lipophilic layer on the outer surface of the plate, then it is the developer which will not be capable of fully removing the coating at the desired areas thereof, when the sensitive coating is directly applied on the hydrophilic surface of the plate.
The solution that has been given to such a problem of incapability of removal of the cured sensitive coating, in accordance with the prior art, is to sell the presensitized plates with the coating only partially cured or fully non-cured, thereby forcing the consumer to effect his own curing operation after developing the plate, when the coating is desired to form part of the lithographic pair, or else to accept a considerably lower fidelity. This, of course, causes serious drawbacks in the control of the curing operation of the coating and very frequently produces defective plates.
On the other hand, while in the market there are planographic plates the lipophilic layer of which is the outer layer and consequently do not offer serious difficulties in connection with the above described problem, inasmuch as the residues of the coating left by the developer at the areas affected thereby are thereafter removed by entrainment with the etching fluid, and on the other hand the residues left by the remover, if the latter is used, at the non-affected areas, by being lipophilic, are irrelevant, it has been found that adherence of the light sensitive layers or coating to said plates is not sufficiently efficient, whereby this very frequently produces a low dot fidelity when the plate is etched.
It is also well known that certain light sensitive coatings are not compatible with hydrophilic surfaces such as chromium or aluminum, whereby it has been necessary to use adhesives such as the commercial product Accobond sold by American Cyanamid, which renders production of said plates more expensive without any benefit in the operation of the plate.
Certain coatings do not show in themselves the necessary contrast against the normal working layers (chromium, copper, aluminum), whereby the prior art plates have required the addition of a pigment or dye to the coating, thereby reducing to a certain extent its sensitiveness to light in the photolytic process, consequently increasing the exposure time, which may even reach impractical levels.
it is also well known that the more commonly used hydrophilic metals for the preparation of presensitized planographic printing plates, such as chromium and aluminum, have a tendency to be oxidized and to be passivated very easily when they are not suitably protected, which brings about a considerable decrease in their hydrophilic power from the time of their electrodeposition to the time of application of the necessary light sensitive coating thereon. Therefore, it has always been regarded as absolutely inappropriate to permit that said metals be exposed to atmospheric conditions, whereby it has been required to protect them by means of layers of gum or the like or to varnish them practically immediately to avoid oxidation, which sometimes constitutes on unsurmontable disadvantage.
The above drawbacks of the prior art presensitized plates, therefore, rendered it impossible to manufacture a highly economical presensitized plate, inasmuch as heretofore it has been necessary, to provide an economical plate, in the case of, for instance, aluminum plates to which the presensitizing coating is applied, to have resort to the use or an organic adhesive for said coating or to anodization or graining processes, with the disadvantage that said plates have an unduly increased cost and a very short life and can only be used for short runs, because it is the light sensitive coating which, once developed and fixed, serves as an ink attracting layer in itself, while the metallic surface of the supporting plate serves as the ink repellent layer, whereby it will be apparent that, an organic light sensitive coating, as hard as it may be, being an organic material, cannot have a reasonably long life, which has left much to desire in connection with the production of low cost presensitized plates for wide utilization. Also, in this type of plates it is not possible to cure the coating up to the optimal point, inasmuch as the developer will leave residues on the hydrophilic aluminum surface, which might produce a deficient printing, whereby it is required that the user be the one to effect the final curing of the coating after developing the same, if a longer life is desired for the plate in use.
On the other hand, this type of drawbacks caused by the lack of adherence of the presensitizing coating on chromium layers, has rendered it impossible heretofore to economically manufacture a multimetallic positive planographic printing plate which may be presensitized with a light sensitive coating and which may have reasonable cost characteristics, inasmuch as in this particular instance it is also necessary to apply a relatively costly adhesive layer, which is also of difficult application and sometimes of low adhesive efficiency, thereby making difficult the manufacture of high quality planographic printing plates, presensitized with a light sensitive coating which may be reliable and which will enable said plates to be etched with a sufficiently high accuracy to be considered as high quality printing plates, as well as to be treated with the remover without leaving residues on the hydrophilic layer after treatment with said remover.